| Electronics > Repair |
| (FIXED) Keithley 225 (current source) repair |
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| motocoder:
Well, was all working! But then while I was trying calibrate it, R105 went up in smoke. I figured those current-limiting transistors had gone out. Checked and sure enough, Q103 was bad. Replaced Q103 and Q106 with beefier ZTX450/550 parts, put it all back together, and it burned through another R105. Quick check of output transistors and I think they might have gone too Output transistors are still good. Not sure what is causing this. |
| cncjerry:
I cant see how the base resistor r105 would burn out unless q105 is shorted. also, wrt my earlier note, there is a plug with the reference 'see dwg 24203e...' on it. if you pull that plug you can diode check all those transistors possibly from the plug itself. one other thing on r105, did you scope it after changing the transistors to another type? Maybe it is oscillating. Current sources are known to have problems with certain reactive loads. |
| motocoder:
--- Quote from: cncjerry on June 05, 2015, 07:21:33 am ---I cant see how the base resistor r105 would burn out unless q105 is shorted. also, wrt my earlier note, there is a plug with the reference 'see dwg 24203e...' on it. if you pull that plug you can diode check all those transistors possibly from the plug itself. one other thing on r105, did you scope it after changing the transistors to another type? Maybe it is oscillating. Current sources are known to have problems with certain reactive loads. --- End quote --- I pulled Q105 out of the socket, and it is fine (not shorted). I checked Q101 as well while I was there. What's interesting is that the negative polarity is not working at all anymore. It did work before the R105 problem. I don't have a thermal camera, but using one of those little infrared thermometers, it appears to me that Q104 is heating up quite a bit. Q104 has been replaced with a 2N5416S, whereas Q102 (the symmetrical/complimentary transistor on the positive side) is still the original part (MM3003). Maybe the overall gain between the two polarities is too mismatched now? Regarding your comment about plug dwg 24203e - there is no plug anywhere on the board. What you're referring to is just how they've split the schematic into two parts. There is no physical connector between the two parts. |
| motocoder:
Something is definitely fishy here. I measures the voltage across R105 and it was ~55V. THen just a few minutes ago, before connecting my scope to it, I measured again, and it's just a few 10's of millivolts. And there is a fair amount of noise on the Vbe of Q101 (~60mV), and to a lesser extent Q105, so there is some oscillation looks like. Also, I can definitely see that Q104 is heating up, even though when I measure Vbe across it, it is just a few tenths of a volt, and Vce is the full 130V. Maybe enough leakage current there that with the high Vce value it generates some heat. |
| cncjerry:
did r105 pop again? Sometimes the burn out and don't flame. I don't remember the value, but do the power math at 55v to be sure it isn't too much which would point you somewhere. when I had a resistor popping in my keithley 228 VI source, one cool product by the way, I put 1/4w in to make it easier to blow. I'm going to go look at the schematic again. |
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